(Space.com) New measurements of the expansion rate of the universe lend new support for the theory of dark energy that suggests a mysterious force is pulling the cosmos apart at ever-increasing speeds.
Scientists have few ideas why such a force would exist, but the evidence for dark energy - which like dark matter has remained elusive to detection attempts -is growing, and a competing hypothesis can apparently be ruled out.

In a new study, a team of researchers led by Adam Riess of the Space Telescope Science Institute in Baltimore, report that they've calculated how fast the universe is expanding to a greater degree of accuracy than ever before, shrinking the error bars on their measurements by about 30 percent.

Hubble telescope measures the universe

The improved accuracy comes thanks to the Wide Field Camera 3 on the Hubble Space Telescope. It's a new instrument that was only installed on the orbiting observatory in 2009, during NASA's last space shuttle mission to upgrade the space telescope.

"Without the improvement in efficiency capability from the new camera, it just wouldn't have been feasible," Riess told SPACE.com Monday (March 14). "It's just a different generation of technology than the previous camera."

The researchers used the new camera to observe a special class of exploding stars called type 1a supernovas, which are useful because they always release the same amount of light. Astronomers compare this intrinsic brightness to their measured brightness - which varies depending on how far away from Earth they are - to judge cosmic distances.

The new measurements confirm astronomers' growing consensus that the universe is not only expanding like a balloon, but picking up speed in doing so.

Based on our current laws of gravity - namely, Einstein's theory of general relativity - that's unexpected. To explain this surprising reality, scientists came up with dark energy, which would counteract the force of gravity that tries to pull the universe inward. But to date, dark energy has remained undetectable to astronomers.

Dark energy opponents

Not all experts are comfortable with the idea that a strange force is mysteriously tugging the universe apart.

"Theorists have come up with very creative ways to get out of dark energy, which would be great because we don't understand dark energy very well and it would be nice to find a way that the universe was simpler," Riess said.

One such alternative is the idea that our cosmic neighborhood - the solar system and the whole Milky Way galaxy - happens to sit at the center of a relatively empty bubble of space eight billion light-years across.

If this were the case, we would measure the same accelerated expansion rate we do, except it would be an illusion created by our special position in the void.

But the new precision measurements of the universe's expansion seem to rule out that idea, which predicts a somewhat different value for the expansion rate.

"This new paper by Riess et al. reduces the errors on the measurement of [the expansion rate], and so improves even further the already high confidence that one should have in saying that these models are ruled out," said Timothy Clifton, an astrophysicist at the University of Oxford who was not involved in the new research. "In this sense, they improve the evidence for the existence of dark energy."

However, Clifton cautioned that the new measurements do not disqualify all versions of the void model. In some more complicated scenarios in which the big bang did not happen at the same time at all points in space, this hypothesis could still be valid.

Ultimately, Riess said many scientists are dubious of all the void models anyway.

"I know that a lot of people have not taken that theory very seriously because of a major problem with it," he said. "We tend to believe theories where we don't live in any special place in the universe. That would be very strange - why should we be in a special place?"

Now that scenario is even less likely to be true, Riess said.

"But on the other hand, dark energy's pretty weird too," he said.

However, it looks like we may be stuck with it.

Riess and colleagues report their findings in the April 1 issue of The Astrophysical Journal.